Emotionally expressive behavior normally undergoes developmental change between infancy and adulthood, as the involuntary, intense expressiveness of infants becomes the more subdued, socially-controlled expressiveness of older individuals. We currently know very little about how these changes come about. However, a small number of studies indicate that the socialization of emotional expressions begins in the first year of infancy. The immediate goal of the present research is to identify developmental changes in the smiling behavior of infants during their first year. The long-range goal is to discover when and how emotional behavior comes under social control, and whether such social learning affects the experience of emotion as well as its expression. By increasing our understanding of normal age-related changes in emotional expression and/or experience, this research will also provide new insights into abnormalities or failures in the socialization of emotion. The particular focus of this first project is on the emergence of the behavior described as "affective sharing" during infancy. In positive affective sharing, the infant smiles for some non-social reason (e.g., during solitary play) but shows the smile to a social object (e.g., mother). Affective sharing has been positively linked to security in the infant-mother relationship. In the present project, five experiments measure the effects of the presence, attentiveness and familiarity of nearby social objects (mother and friendly stranger) on the frequencies and physical forms of smiles produced by 4-, 6-, and 8-month-old infants during non-social play. The social objects' behaviors in response to physically different infant smiles are also analyzed and compared. In addition, one short-term longitudinal study seeks to account for our previous finding that anticipatory smiling emerges between 8 and 10 months of age. The findings will provide unique information about the origins of social control over smiling, and will give direction to future research on the role of affective sharing in social/emotional development.